XVI
ON THE STRAIN THEORY OF PHOTOGRAPHIC ACTION
The uncertainty in regard to satisfactory explanation of photographic action is due to the experimental difficulties in studying the problem. As for instance:—
(1) There is reason to believe that every substance is molecularly affected by radiation, but detection of the induced change is rendered impossible by the imperfection of methods hitherto available, and also by the subsequent self-recovery of the substance in darkness. The effects can be detected in a few cases only when the changes produced happen to be visible, or become visible on development.
(2) As regards direct chemical tests, taking for example, the case of AgCl, the quantity of radiation product is exceedingly small; this occurs in the presence of a very large amount of unchanged chloride and the isolation of the minute traces of changed product is extremely difficult. Various secondary reactions, moreover, complicate the phenomenon.
To arrive at a correct idea of the changes produced, it is necessary to measure the minute effects produced by radiation on the extremely thin layer—perhaps only a few molecules deep—of the sensitive substance. In order to ascertain this, it is desirable to begin with the study of some elementary substance in which its effects are attended with few secondary complications. And,
(208)
